Investigation of the pathways for biosynthesis of phosphatidylcholine has been extended to two plants in addition to the Lemna previously studied, and to rat liver and hepatoma cells. Special attention was focussed upon the role of phosphoethanolamine derivatives by use of methods which had demonstrated the participation of such derivatives in Lemna. Surprisingly, each of the plant tissues uses different steps to carry out the methylation reactions involved, although methylation of phosphoethanolamine appears to be the committing step common to each plant. Choline, or a metabolite thereof, markedly down- regulates entry of methyl groups into the network of methylated ethanolamine derivatives in each of the three tissues. Rat liver uses a pathway which is gain different in which all three methylations take place at the phosphatidyl-base level. In hepatoma cells it was demonstrated that entry of methyl groups into this pathway is regulated also by the availability of choline.